This invention relates to bobbins, transformers, magnetic components, and methods.
FIGS. 1A and 1B show, respectively, a top and side view of a transformer 10 of the kind described in U.S. Pat. No. 5,719,544 (xe2x80x9cTransformer With Controlled Interwinding Coupling and Controlled Leakage Inductances and Circuit Using Such Transformer,xe2x80x9d Vinciarelli et al., assigned to the same assignee as this application and incorporated herein by reference, the xe2x80x9ctransformer patentxe2x80x9d). The transformer comprises two bobbin assemblies 1A, 1B, each comprising an electrically conductive winding 2A, 2B wound over a non-conductive bobbin 4A, 4B. The two windings are linked by a magnetic medium comprising two core assemblies 11. Each core assembly comprises an electrically conductive medium 12 selectively arranged over the surface of a permeable core piece 6 (e.g., by means of platingxe2x80x94see, for example, U.S. patent application Ser. No. 08/941,219 filed on Oct. 1, 1997xe2x80x94or use of formed sheets or foils). The faces 8 of the core pieces 6 are free of conductive medium and a slit is provided along the inner periphery of the core assemblies (not shown), thereby preventing formation of a xe2x80x9cshorted turn.xe2x80x9d The conductive medium 12 constrains the transformer leakage flux to lie within the region confined by the conductive medium. As discussed in the transformer patent, such a transformer has a number of benefits: it exhibits much lower leakage inductance than similar transformers without a conductive medium; the widely separated windings exhibit low interwinding capacitances; the placement of the windings provides for easy removal of heat; and many different transformers, varying in terms of turns ratio and leakage inductance, may be inductance of the transformer may be set by means of a gap 16 in the magnetic path (a portion of the bobbin 4B and winding 2B are shown cut away to show the gap).
In other transformer embodiments, described in the transformer patent and shown in FIG. 2, extensions 20 of the permeable magnetic material may be used to provide a low reluctance path for leakage flux 21 in the region between the core halves, thereby providing a greater possible range of leakage inductance. Such extensions 20 may also be covered with a conductive medium.
As shown in FIG. 3, a saturable inductor 22 is sometimes placed in series with a winding 26 of a transformer 24 in a switching power supply. In some applications, the saturable inductor is used to limit rectifier 32, 33 reverse recovery currents and attendant conducted and radiated noise. Such an inductor may also be used in a converter comprising an xe2x80x9cactive clampxe2x80x9d core resetting circuit 30 (of the kind described in U.S. Pat No. 4,441,146, xe2x80x9cOptimal Resetting of the Transformer""s Core in Single-Ended Forward Converter, Vinciarelli, assigned to the same assignee as this application, incorporated by reference) to provide a high impedance load on the transformer winding for a short time following turn-on of the main switch 28, thereby allowing the xe2x80x9cmirroredxe2x80x9d flow of transformer magnetizing current to more fully charge and discharge parasitic capacitances than would otherwise be possible without it and allow for zero-voltage switching operation. The number of turns on the saturable inductor 22 will depend on the required xe2x80x9cvolt-secondxe2x80x9d rating and will, for a given transformer configuration, vary as a function of the output voltage of the converter. To maintain a fixed xe2x80x9ctime to saturationxe2x80x9d, the number of turns on a saturable inductor will, for a given saturable core, need to increase in proportion to transformer output voltage. Thus, different saturable inductors are generally required for different output voltage settings.
In general, in another aspect, the invention features a leakage inductance transformer that includes a bobbin, a winding surrounding the bobbin, a permeable magnetic core having a magnetically permeable segment which passes within the bobbin to form a flux path that couples the winding, and a permeable magnetic insert that is located outside of a hollow interior space enclosed by the bobbin.
Implementations of the invention may include one or more of the following features. The bobbin may have an electrically insulating wall surrounding a hollow interior space, the electrically insulating wall including segments having different thermal conductivities to provide the confined thermally conductive channel. The confined thermally conductive channel may be provided by ceramic (e.g., alumina). One of the segments may be plastic. A solderable metal coating of the bobbin may provide the confined thermally conductive channel and may be attached to the permeable core. The confined thermally conductive channel may have a thermal conductivity greater than 1 BTU/(hourxfootxdeg.F) while another segment of the bobbin may have a thermal conductivity less than 1 BTU/(hourxfootxdeg.F)).
A magnetically permeable insert strip of amorphous magnetic material may be located outside of a hollow interior space enclosed by the bobbin. The insert may lie in a flux path defined by, and be permeably linked to, the leakage lug. The insert may be a saturable magnetic material. The insert may lie in a plane perpendicular to the thermally conductive wall of the bobbin.
Other advantages and features will become apparent from the following description and from the claims.